1. Field of Endeavor
The present invention refers to the field of power generation. It relates to a combined cycle power plant and to a method for operating a combined cycle power plant.
2. Brief Description of the Related Art
The comparatively low CO2 content in the exhaust gas of a gas turbine is disadvantageous if it involves the separation of CO2 from the exhaust gas. It is known from the prior art (see for example publications EP-A2-1 752 616 or DE-T5-102 97 365), in combined cycle power plants with heat recovery steam generators, to return exhaust gases which come from the gas turbine to the inlet of the gas turbine, after flowing through the heat recovery steam generator, in order to both increase the CO2 content and to reduce the mass flow which has to be processed in the devices for CO2 separation. At the outlet of the heat recovery steam generator the exhaust gas has a temperature of approximately 100° C. which then has to be reduced to 50° C. or less for the separation of the CO2.
The simplest sequential arrangement in principle of the system which is reproduced in FIG. 1 (wherein the supplementary firing 17, which is integrated in the heat recovery steam generator 16, with the fuel feed line 30, has to be imagined as not being included in the system in order to illustrate a prior system, as FIG. 1 illustrates an exemplary embodiment of the present invention) is adequate for a combined cycle power plant with exhaust gas recycling. The combined cycle power plant 10 of FIG. 1 has a gas turbine 11 with a compressor 12, a single-stage or multistage combustion chamber 14 with a corresponding fuel feed line 34, and a turbine 13, and also a heat recovery steam generator 16 which is connected downstream and converts feed water which is fed via a feed-water inlet into steam which flows via one or more live steam outlets 18 to a steam turbine 19. Air is drawn in by the compressor 12 via the air inlet 15 and compressed. With the compressed air, the gaseous and/or liquid fuel, which is fed via a fuel feed line 34, is combusted in the combustion chamber 14. The hot exhaust gases are expanded in the turbine 13, performing work, and then guided through the heat recovery steam generator 16 for producing steam for the associated water/steam cycle 36. The exhaust gas which discharges from the heat recovery steam generator 16 is cooled in a first heat exchanger 22 and then divided into two flows. The one flow is returned via an exhaust gas recycling line 28 to the inlet of the gas turbine 11 and, in doing so, is cooled down further in a second heat exchanger 29. The other flow is compressed by a compressor 24 and guided to a CO2 separating plant 25 where the CO2 is largely separated from the exhaust gas. The separated CO2 is discharged via a CO2 outlet 27, and the exhaust gas which is largely freed of CO2 is discharged via an exhaust gas line 26. The steam, which is required for the separation, is extracted at the steam turbine 19 or at the heat recovery steam generator 16 and fed to the CO2 separating plant 25 via a steam extraction line 23. The accumulating condensate of the CO2 separating plant 25 is fed again via a condensate return line 35 to the water/steam cycle 36 of the combined plant. For CO2 separating plants 25 without heat requirement the steam extraction line 23, as well as the condensate return line 35, can be dispensed with, as is shown in FIG. 1a. 
An exhaust gas bypass 21, via which in specific cases the exhaust gas can be discharged directly to the atmosphere, can be provided between the heat recovery steam generator 16 and the subsequent heat exchanger 22.
In the case of this type of augmented concentration of CO2, the high equipment cost of exhaust gas ducts and heat exchangers which is associated with the recycled exhaust gas flow at a recirculation rate of about 50%, and also the reduction of generated output which is associated with the CO2 separation, is disadvantageous.